help from the community?

Simon Peyton-Jones simonpj at
Tue Jan 30 11:27:18 EST 2007

| > I can also imagine predicates that do not mention locally-quantified
| > variables - the assumption must be that they mention variables bound on
| > the LHS of the datatype decl instead?  e.g. the Show predicate here:
| >
| >     data Foo a b = Foo a b
| >                  | Bar (forall c . (Show b, Relation b c) => (b,c))
| >
| > Hmm, maybe a simpler version of this example would illustrate what you
| > mean by the proposal (first of the three bullets) to allow an empty
| > quantifier list:
| >
| >     data Foo a b = Foo a b
| >                  | Bar (forall . Show b => b)
| >
| > In which case, does this even count as a polymorphic component at all?
| > Is it not rather GADT-like instead?
| >
| >     data Foo a b where
| >       Foo :: a -> b -> Foo a b
| >       Bar :: Show b => b -> Foo a b
| I was thinking that we should allow those special cases because I
| could not see a reason to disallow them (rather then having a
| compelling example to use them).   You make a good point though, that
| some of them might indicate an error in the program.  So, I guess, the
| main decision is: do we want to make them illegal (i.e., require an
| error) or suggest that implementations report a warning?  I have no
| strong feelings either way, but I guess we need to pick something.

I think I know what we should do on this particular point.  I've even documented it here:

I would like to urge this design, or one close to it, for Haskell'.  Note that this is *not* the same as adopting GADTs.


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